[libc] Allow reading and writing __FE_DENORM if available on x86_64.

[sivachandra]

Summary

Some libcs define __FE_DENORM on x86_64. This change allows reading the
bits corresponding to that non-standard exception.

libc/utils/FPUtil/x86_64/FEnv.h

 
 static constexpr uint16_t X87RoundingControlBitPosition = 10;
 static constexpr uint16_t MXCSRRoundingControlBitPosition = 13;
 
 // The exception flags in the x87 status register and the MXCSR have the same
 // encoding as well as the same bit positions.
 struct ExceptionFlags {
   static constexpr uint16_t Invalid = 0x1;
-  static constexpr uint16_t Denormal = 0x2; // This flag is not used
+  // Some libcs define __FE_DENORM corresponding to the denormal input
+  // exception and include it in FE_ALL_EXCEPTS. We define and use it to
+  // support compiling against headers provided by such libcs.
+  static constexpr uint16_t Denormal = 0x2;
   static constexpr uint16_t DivByZero = 0x4;
   static constexpr uint16_t Overflow = 0x8;
   static constexpr uint16_t Underflow = 0x10;
   static constexpr uint16_t Inexact = 0x20;
 };
 
 // The exception control bits occupy six bits, one bit for each exception.
 // In the x87 control word, they occupy the first 6 bits. In the MXCSR
 // register, they occupy bits 7 to 12.
 static constexpr uint16_t X87ExceptionControlBitPosition = 0;
 static constexpr uint16_t MXCSRExceptionContolBitPoistion = 7;
 
 // Exception flags are individual bits in the corresponding registers.
 // So, we just OR the bit values to get the full set of exceptions.
 static inline uint16_t getStatusValueForExcept(int excepts) {
   // We will make use of the fact that exception control bits are single
   // bit flags in the control registers.
   return (excepts & FE_INVALID ? ExceptionFlags::Invalid : 0) |
+#ifdef __FE_DENORM
+         (excepts & __FE_DENORM ? ExceptionFalgs::Denormal : 0) |
+#endif // __FE_DENORM
          (excepts & FE_DIVBYZERO ? ExceptionFlags::DivByZero : 0) |
          (excepts & FE_OVERFLOW ? ExceptionFlags::Overflow : 0) |
          (excepts & FE_UNDERFLOW ? ExceptionFlags::Underflow : 0) |
          (excepts & FE_INEXACT ? ExceptionFlags::Inexact : 0);
 }
 
 static inline int exceptionStatusToMacro(uint16_t status) {
   return (status & ExceptionFlags::Invalid ? FE_INVALID : 0) |
+#ifdef __FE_DENORM
+         (status & ExceptionFalgs::Denormal ? __FE_DENORM : 0) |
+#endif // __FE_DENORM
          (status & ExceptionFlags::DivByZero ? FE_DIVBYZERO : 0) |
          (status & ExceptionFlags::Overflow ? FE_OVERFLOW : 0) |
          (status & ExceptionFlags::Underflow ? FE_UNDERFLOW : 0) |
          (status & ExceptionFlags::Inexact ? FE_INEXACT : 0);
 }
 
 struct X87StateDescriptor {
   uint16_t ControlWord;